Although the raphe-serotonin system of the mammalian CNS has been implicated in a variety of behavioral and physiological processes such as sleep, sexual behavior, and temperature regulation, most of these data are indirect. The present proposal is aimed at directly examining the factors that control raphe unit activity and the functional role of these neurons both at the cellular and the behavioral level. The following specific studies are proposed. Utilizing intracellular and extracellular unit recordings in vitro (400 micron thick slabs of rat brain tissue) we will examine whether the slow rhythmic activity of raphe neurons is mediated by synaptic inputs from other neurons, or whether the generation of this patterned activity is endogenous to raphe neurons. Extracellular single unit activity will be studied in chloral hydrate anesthetized rats in order to: examine whether there is a neuronal feedback mechanism in the serotonin system as there is in the brain dopamine system; examine the effect of stimulating various forebrain sites on raphe unit activity (these studies will be aided by horseradish peroxidase studies of raphe afferents). We will also study the activity of single raphe neurons in relation to behavioral states, especially sleep, in the free-moving rat. Studies of unit activity in the ventral lateral geniculate nucleus will examine the influence of serotonin on neuronal coding in the visual system. Finally, utilizing discrete electrolytic lesions of specific raphe nuclei or the specific neurotoxic compound 5,7-dihydroxytryptamine, we will re-examine the role of serotonin in sleep (lesion placements will be aided by fluorescence histochemical studies). The experiments are tied together by an hypothesis that relates the phenomenological similarity of REM sleep and drug-induced hallucinatory states to the role of the midbrain raphe as a visual system modulator.